1. Field of the Invention
The present invention relates to a drive transmitting apparatus for selectively transmitting a driving force from a drive source to a driving member through clutch means and an image forming apparatus using such a drive transmitting apparatus.
2. Related Background Art
Among drive transmitting mechanisms for image forming apparatuses, there is a mechanism in which a partially non-toothed gear having a non-toothed portion is used as clutch means for effecting connection and disconnection of a driving force, as disclosed in Japanese Patent Application Laid-open No. 10-274312. According to this mechanism, in a normal condition, the non-toothed portion is opposed to a drive gear to block transmission of the driving force to the partially non-toothed gear, and, when the partially non-toothed gear is slightly rotated by trigger means such as a solenoid, the drive gear is engaged by the partially non-toothed gear, thereby transmitting the driving force to the partially non-toothed gear. When the partially non-toothed gear is rotated by one revolution, since the transmission of the driving force is blocked again by the non-toothed portion, this mechanism acts as a one revolution controlling clutch.
On the other hand, as disclosed in Japanese Patent Application Laid-open No. 9-141966, there is an apparatus in which a driving force is switched by connecting a disconnecting clutch means of a driving system by utilizing a movement of a carriage. In this apparatus, a pair of clutch gears having triangular teeth (ratchets) formed on opposed side surfaces and engageable with each other are provided, and, when one of the clutch gears is pushed toward the other clutch gear by the carriage, the triangular teeth are engaged by each other, thereby transmitting a driving force. According to this arrangement, a construction in which one of the clutch gears is pushed by using a solenoid is not required, and trigger for transmitting the driving force can be achieved by using the movement of the carriage itself, thereby reducing the cost.
However, in the above-mentioned conventional technique, when the drive gear begins to be engaged by the partially non-toothed gear, the tops of the teeth may abut against each other to cause poor engagement, with the result that noise may be generated or an out-of-phase condition of a motor may be caused. In the technique disclosed in Japanese Patent Application Laid-open No. 10-274312, although special tooth configurations in which inclined surfaces are formed on the teeth are adopted to avoid such inconvenience, it is difficult to set the optimum tooth configurations.
On the other hand, in the technique disclosed in Japanese Patent Application Laid-open No. 9-141966, as shown in FIG. 9, a trigger tooth portion 901b complementary to the non-toothed portion is provided aside the non-toothed portion 901a of the partially non-toothed gear 901, and the drive gear 902 is opposed to the non-toothed portion 901a and a driven gear 903 is disposed in an engagement relationship to the trigger tooth portion 901b. 
The driven gear 903 has no self-driving ability and can be moved in a thrust direction so as to contact with and separate from the drive gear 902. When the driven gear 903 is urged against the drive gear 902, the opposed triangular teeth (not shown) engageable with each other are meshed with each other, thereby transmitting the driving force of the drive gear 902 to the driven gear 903. As a result, the driving force from the drive gear 902 is transmitted to the partially non-toothed gear 901 through the driven gear 903, thereby starting rotation of the partially non-toothed gear 901. When the partially non-toothed gear 901 is rotated by a predetermined amount, the drive gear 902 is engaged by a toothed portion 901c of the partially non-toothed gear 901, with the result that, even when the urging of the driven gear 903 against the drive gear 902 is released, the driving force can be transmitted. In this case, so long as the phases of the gear portions of the gears 902, 903 in a condition that the triangular teeth are engaged by each other are aligned with each other, since the tooth of the drive gear 902 is smoothly engaged by the toothed portion 901c of the partially non-toothed gear 901, the tops of the teeth of both gears are prevented from abutting against each other to cause the poor engagement.
However, immediately before the partially non-toothed gear 901 is rotated by one revolution to return to its initial position again, when the driven gear 903 freely rotatable without transmission of the driving force due to disconnection of the triangular tooth from the triangular tooth of the drive gear 903 is engaged by the trigger tooth portion 901b again, the tops of the teeth of the driven gear 903 and the trigger tooth portion 901b may abut against each other to cause poor engagement.
In this case, so long as a condition that the driven gear 903 is urged against the drive gear 902 is maintained while the partially non-toothed gear 901 is rotated by one revolution, since the phases of the gears can be aligned even when the driven gear 903 and the trigger tooth portion 901b are engaged by each other again, this problem can be solved. However, meanwhile, the carriage must be retained in a predetermined position for urging the driven gear 903 against the drive gear 902, thereby delaying the further recording operation.
The present invention aims to eliminate the above-mentioned conventional drawbacks, and an object of the present invention is to provide a drive transmitting apparatus in which a positive switching operation can be achieved without noise and/or poor drive transmission from a drive source, and an image forming apparatus using such a drive transmitting apparatus.
Another object of the present invention is to provide a drive transmitting apparatus comprising an output gear having a non-toothed portion not including a predetermined number of teeth and an entirely toothed portion having no non-toothed portion in a tooth width-wise direction, an input gear disposed at a position where it can be opposed to the non-toothed portion of the output gear and rotated by a driving force from a drive source, a trigger gear meshed with the entirely toothed portion of the output gear and rotatable with respect to a rotary shaft and movable in a thrust direction so as to contact with and separate from the output gear, and engagement portions provided on opposed side surfaces of the input gear and the trigger gear and engageable with each other, and wherein, when the input gear and the trigger gear are engaged by each other through the engagement portions, the driving force from the drive source is selectively transmitted to a driving member through clutch means for synchronizing phases of the input gear and the trigger gear, and an image forming apparatus using such a drive transmitting apparatus.
A further object of the present invention is to provide a drive transmitting apparatus in which a positive switching operation can be achieved without noise and/or poor drive transmission from a drive source by providing a construction in which tops of teeth of an input gear and an output gear do not abut against each other by aligning phases of engagement portions of the input gear and a trigger gear, and an image forming apparatus using such a drive transmitting apparatus.
A still further object of the present invention is to provide a drive transmitting apparatus in which engagement portions of two gears are constituted by ratchets having relative symmetrical configurations and engageable with each other so that, when phases of two gears are aligned, a gap is created between the ratchets.